1. Field of the Invention
The present invention relates generally to a power transmission apparatus, and more specifically to a power transmission apparatus of viscous coupling differential gear type suitable for use in an automotive vehicle.
2. Description of the Prior Art
A typical prior-art power transmission apparatus of viscous coupling differential type will be described hereinbelow with reference to FIG. 1. In the drawing, a power transmission apparatus 100 roughly comprises a differential gear casing 101 driven by an appropriate ring gear (not shown), a pinion gear 105 rotatably supported by a pinion shaft 103 fixed to the differential casing 101, a first side gear 109 in mesh with the pinion gear 105 and spline-engaged with a first output shaft 107, a second side gear 113 also in mesh with the pinion gear 105 and spline-engaged with a second output shaft 111.
In the case of an automotive vehicle, the first output shaft 107 is a rear left side wheel shaft, while the second output shaft 111 is a rear right side wheel shaft, for instance. Therefore, when the automotive vehicle travels along a straight road, the two first and second side gears 109 and 113 rotate at substantially the same speed without rotating the pinion gear 105. On the other hand, when the automotive vehicle travels along a curved road, the pinion gear 105 rotates about the shafts 107 or 111 rotational speed between the two output (wheel) shafts 107 and 111.
In addition, the power transmission apparatus 100 further comprises a viscous coupling device (VCD) 125 provided between the differential gear casing 101 and the first output shaft 107 to limit the differential operation between the two output shafts 107 and 111.
In more detail, an inner hub 115 is spline engaged with the first output shaft 107, and a number of first resistance plates 117 are spline engaged with the inner hub 115 at regular axial intervals. On the other hand, a coupling case 121 is fixed to the differential gear casing 101 with screws 121A, and a number of second resistance plates 119 are spline engaged with the coupling case 121 at regular axial intervals. Each of the first resistance plates 117 (fixed to the first output shaft 107) and each of the second resistance plates 119 (fixed to the differential gear casing 101) are arranged alternately within a closed working chamber 123 filled with a viscous fluid.
In the prior-art power transmission apparatus as described above, however, when a tire is punctured and therefore a spare (temper) tire (whose diameter is a little smaller than that of the ordinary tire) is replaced with the punctured tire an one wheel side or when tire inflating pressure is different from each other between both side wheels, a difference in rotational speed is kept produced for many hours between both the wheels. In this case, since the first output shaft 107 rotates at high speed relative to the differential gear casing 101, the viscous fluid within the working chamber 123 is always sheared off by a great number of the resistance plates 117 and 119. As a result, there exists a problem in that the viscous fluid is heated up to an abnormally high temperature and/or high pressure, so that the resistance plates 117 and 119 tend to seize and therefore it is impossible to obtain a smooth differential operation.